A microfilm strip for typical use on a microfilm reader-printer apparatus has recorded thereon a series of image frames arranged at certain intervals longitudinally of the microfilm strip. To provide easy and fast access to such image frames during reading mode of operation of the apparatus, the microfilm strip further has appropriate identification marks or indices respectively allocated to the individual image frames. Such frame identification marks or indices are commonly called "blip marks" and are indicative of the locations of the respectively associated image frames on the microfilm strip. Any desired image frame can be retrieved out of the image frames of the microfilm strip through detection of the blip mark allocated to the particular image frame with the microfilm strip driven for movement with respect to the optical system of the apparatus.
Not all of the microfilm strips presently available are however furnished with such blip marks or any other form of frame identification marks or indices. Microfilm strips devoid of such marks or indices include 35-mm rolled microfilms and some of 16-mm rolls microfilms, as well known in the art. Where such a microfilm strip used on a microfilm reader-printer apparatus, it is required to retrieve a desired image frame through direct detection of the image frames until the particular image frame is accessed. The most basic manner of retrieving the desired image frame is to drive the microfilm strip for forward or backward movement by manual operation while viewing the images reproduced successively on the viewing screen of the apparatus until the desired image frame is reached on the screen. This manner of retrieval relies on the operator's viewing of the viewing screen and requires the operator to concentrate his attention on the viewing screen while manually controlling the movement of the microfilm strip. In addition, time-consuming consuming adjustment and re-adjustment efforts will also be necessitated for the operator so that the desired image frame which has been successfully accessed is correctly reproduced within a prescribed area of the viewing screen.
Another manner of retrieving a desired image frame from a microfilm strip is taught in U.S. Pat. No. 4,453,823 in which is disclosed a microfilm reader-printer apparatus provided with means to detect the margin areas between the adjacent image frames of a microfilm strip. The margin detect means is located close to the path along which a microfilm strip is to travel through the optical system of the apparatus and includes a margin sensor or sensors located in the path of the light which has been transmitted through the microfilm strip. Any desired one of the image frames on the microfilm strip is to be retrieved through detection of the margin areas between successive image frames by the margin detect means thus arranged. A modified version of such margin detect means is known which includes a margin sensor or sensors located in proximity to the location where the beam of light to be incident on the viewing screen of the apparatus is to be focussed.
Frequently used on a microfilm reader-printer apparatus are negative films rather than positive films for some practical reasons which are well known in the art. A negative microfilm has transparent image and margin areas and opaque background areas for each image frame of the film. In response to the transparent image and margin areas of such a negative microfilm strip, the prior-art margin detect means will produce output signals which are such that the signal produced responsive to the image area is close to or may be even equal in signal level to the signal produced responsive to the margin areas. It will be extremely difficult to have one of such nearly equal signal levels correctly discriminated from the other in a signal processing circuit. The signal processing circuit may therefore frequently fail to correctly recognize the signal from the margin detect means so that extremely exacting design considerations will be required to design and organize the sensors implementing the margin detect means and the signal processing circuit to operate on the signals from such sensors.
The present invention contemplates elimination of these and other problems which have thus far been inherent in an image projecting apparatus using prior-art margin detect means of the described natures. Such an object is to be accomplished on the discovery that all the described problems inherent in the prior-art margin detect means result from the arrangement in which the margin detect means is located close to the path of travel of the microfilm strip or in proximity to the location or point at which the beam of light to be finally incident on the viewing screen is to be focussed.